Bicycle and treadmill stress can both be used to perform MCE, though one must be quick to obtain wall motion images (<90 seconds) so that perfusion assessment still occurs whilst the patient has a sufficiently elevated heart rate (for post-treadmill imaging). Both techniques work well within individual countries depending upon the patient population being studied and patient preference.
How to perform exercise MCE – a practical guide
The protocol is similar to conventional stress echocardiography, in that wall motion is acquired in the three apical and two parasternal views at rest and at peak stress. Following this, myocardial perfusion is assessed using low power, real-time MCE and/or high-power, triggered imaging MCE as detailed below:
Above: Protocol for assessment of myocardial perfusion using low MI, real-time imaging (From Senior et al, 2009, Contrast Echocardiography: Evidence –based recommendations)
Above: Protocol for assessment of myocardial perfusion using high MI, triggered imaging (From Senior et al, 2009, Contrast Echocardiography: Evidence –based recommendations)
Both positive inotropes (e.g. dobutamine) and vasodilators (e.g. dipyridamole, adenosine or regadenoson) can be used for assessment of myocardial perfusion. The main advantage of pharmacological stress is the greater time window for acquiring images which often makes this method preferable to exercise stress, especially when first learning MCE.
Dobutamine is a positive inotrope (increases contractility) and positive chronotrope (increases heart rate) and thus increases myocardial work. As with exercise stress, one aims to achieve at least 85% of the maximum predicted heart rate and sometimes atropine (which blocks the parasympathetic nervous system) is also given to help achieve this heart rate target.
Side-effects that can occur during dobutamine infusion include palpitations, chest discomfort, light-headedness and nausea (1 in 300 incidence). Patients may develop atrial fibrillation but this usually returns to sinus rhythm within 24 hours. More serious problems such as ventricular arrhythmias, myocardial infarction and cardiac arrest are extremely uncommon – death is usually quoted at approximately 1/5000 chance.
How to perform dobutamine MCE – A practical guide
The protocol above, taken from the 2009 EAE recommendations on contrast echocardiography, illustrates the dobutamine stress echocardiography protocol. Contrast is given at rest in order to assess wall motion and, if required, perfusion (at rest, normal wall motion MUST imply normal perfusion and thus perfusion assessment is not strictly required, though we recommend it is performed to optimise machine settings so that this does not need to be done at peak stress).
At peak heart rate, low MI real-time imaging and high MI, triggered imaging can be performed using the same MCE protocols shown above.
Dipyridamole / adenosine / regadenoson all cause coronary vasodilation by increasing endogenous adenosine levels. The reason that stresses the heart even though it causes vasodilation is complex and known as the steal phenomenon – there can be horizontal and vertical steal.
Vertical steal (see figure 4) requires presence of a coronary stenosis. When vasodilation occurs after drug administration, the subepicardium steals blood from the subendocardium due to a fall in post-stenotic pressure (which causes a critical fall in subendocardial perfusion pressure).
Horizontal steal (see figure 5) requires the presence of a collateral circulation. The coronary flow reserve (CFR) must be exhausted in the vessel receiving a collateral supply and at least partially preserved in the vessel giving off the collateral supply. In this situation, when arterial vasodilation occurs, the arteriolar bed of the donor vessel now ‘competes’ with that of the receiving vessel. As a result, the vessel with partially preserved CFR receives more blood and the region that was dependent on the collateral circulation is now rendered ischaemic.
Figure 4: Vertical Steal. At rest, perfusion in the circumflex (Cx) is maintained due to vasodilation of the arteriolar bed (larger circles downstream from epicardial vessel), thus using some of the coronary flow reserve (CFR). After dipyridamole-induced vasodilation, flow through the LAD vessel increases significantly (as it can vasodilate normally since no CFR used at rest because there is no stenosis) However, the fall in perfusion pressure through the stenosed artery causes a critical drop in perfusion pressure to the capillary bed downstream, resulting in closing or ‘derecruitment’ of the capillaries. [Modified from Picano E. Stress Echocardiography. 5th edition 2009]
Figure 5: Horizontal Steal. The RCA is donating collateral supply to the diseased LAD. The LAD arterioles are vasodilated at rest (larger vessels drawn on left side of image). After dipyridamole-induced vasodilation, there is a drop in pressure along the supply artery and thus distal perfusion pressure to the collateral vessels falls. The RCA arteriolar bed thus steals blood from the LAD system. [Modified from Picano E. Stress Echocardiography. 5th edition 2009]
Side-effects that can occur during vasodilator infusion include palpitations, chest discomfort, headache and nausea. Arrhythmias are less commonly seen than with dobutamine. Patients are routinely given the adenosine antagonist aminophylline at the end of the test.
How to perform dipyridamole MCE – A practical guide
The protocol above, taken from the 2009 EAE recommendations on contrast echocardiography, illustrates the vasodilator stress echocardiography protocol. For dipyridamole, usually 0.56mg/kg is given intravenously over 4 minutes, though some centres prefer the accelerated high-dose protocol of 0.84mg/kg over 6 minutes.
Contrast-enhanced imaging is performed at rest and 2 minutes after completion of injection of the vasodilator.
One advantage of vasodilator stress is that patients do not usually have a significant tachycardia (unlike dobutamine) or profound respiratory artefact (unlike exercise) and thus imaging is often less technically challenging.
So which stress modality is best?
For the assessment of myocardial perfusion, in theory, it is best to use an agent that causes arteriolar vasodilation. Thus, pure arterial vasodilators (e.g. dipyridamole / adenosine / regadenoson) are considered by many purists the ideal agents for MCE. However, studies have shown that MCE performed with dobutamine (or even exercise, which of course induces vasodilation too) are just as accurate as vasodilator MCE.